A number of methods traditionally have been used to obtain fluid samples from various locations, such as environmental testing wells. One method is the Bennett pump, an air driven pump typically used at depths of less than about 800 feet. The existing Bennett pump technology has been used successfully for several years at the Nevada Test Site (NTS) to collect groundwater samples for geochemical analysis. However Bennett pump use has been limited to a subset of the total number of wells that could be sampled, at the NTS, for example, by the operational limitations of the pump.
Bennett Sample Pumps, Inc. previously manufactured a pump that could lift up to 1,600 feet. However, tubing bundles typically experienced significant problems at these deployment depths.
One difficulty experienced in Bennett pump installation in deep boreholes is associated with the mechanical crushing of the tubing bundle as it tracks around a crown pulley at the surface (FIG. 1). The crushing phenomenon is caused by the weight of the tubing bundle, and associated borehole water in the lift tube, being counteracted by a pull force induced by the tubing spooling winch unit. An 800-foot tubing bundle hanging in a borehole has a weight of approximately 170 pounds dry. When all of the tubes are filled with water, the bundle and associated water has a combined weight of approximately 200 pounds. These weights typically scale with increased borehole depth and tubing length. The problem is exacerbated by the bundle design itself.
The tubing bundle typically used with Bennett pumps is assembled by co-wrapping three individual pieces of polypropylene tubing together with a 3/32″ aircraft cable and a single, multi-conductor electrical cable. These individual components are brought together from individual material spools to form a bundle on the factory floor, and are then packaged by continuously wrapping with polyvinyl chloride pipe wrap tape before being wound onto a shipping spool. This bundle has an effective diameter of 1.8 inches. Because of this significant diameter, and because of the weight of the bundle hanging in the borehole counteracted by a winch pullout force, the bundle tends to crush, or flatten, as it travels around the crown pulley. The subsequent distortion of the bundle leads to a physical, permanent crushing of individual tubes in the bundle and to delamination of the tape wrap.
Repairs to crushed sections of Bennett tubing bundles are not typically satisfactory because repair hardware results in a localized increase in the Bennett tubing bundle diameter, which can lead to difficulties in the repair traveling through the crown pulley. Recurrent leaks and the associated difficulty in locating leaks under a continuous tape wrap also contribute to the unsuccessful repair and re-use of damaged Bennett tubing bundles. There are reported examples of tubing bundles being crushed beyond repair during the first-use cycle. There is one reported case of the tubing bundle being crushed to such a degree during the initial stages of removal that extreme difficulties were encountered in removing the bundle successfully from the borehole, thereby putting the integrity and long-term accessibility of the borehole at risk.
Additional problems and complications can arise from simply coupling together multiple pumps in an attempt to remove liquid from deeper locations. For example, one prior attempt to couple two Bennet pumps pumped both water and air. The air was produced as a consequence of cavitation.